OBJECTIVES: To study the efficacy, safety, and tolerability of the 300 microg dose of a new chromatographically produced rhesus immunoglobulin (Rhophylac 300) for ante- and postnatal rhesus prophylaxis. DESIGN: In an open-label multi-centre study, rhesus D (RhD)-negative women were randomly allocated to receive Rhophylac 300 either intravenously or intramuscularly at the 28th week of gestation and within 72 h after delivery of an RhD-positive child. Serum samples were obtained prior to the antenatal dose and 6-11.5 months after delivery of an RhD-positive child and tested by the indirect antiglobulin test and papain test for anti-D. Safety parameters were assessed in all women who were treated with the study drug. RESULTS: Four hundred and thirty two women received the study drug antenatally. No differences were detected in efficacy or tolerability between intravenous and intramuscular administration. Of the 261 women who delivered an RhD-positive child and received rhesus prophylaxis according to the protocol, 248 women returned for follow-up investigations. None of them had detectable anti-D at their last visit. There were no serious adverse events, no cases of infectious disease transmission nor clinically relevant changes in laboratory safety values and vital signs attributable to the study drug. CONCLUSIONS: The results suggest that Rhophylac 300 given intravenously or intramuscularly is safe and efficacious in preventing rhesus (D) immunisation.